This invention relates to methods and apparatus for feeding and continuously casting molten metal for continuously casting metal strip, sheet, slab, plates, bars, or billets directly from molten metal introduced through a semi-sealing nosepiece into the casting region of a moving mold between spaced portions of two moving cooling surfaces which cool the metal being cast.
The invention herein is described as embodied in the structure and operation of casting machines in which the molten metal is fed through a semi-sealing nosepiece into the moving mold or casting region located between opposed portions of two moving water or liquid-cooled molds having surfaces defining the mold region. The moving molds in the illustrative examples shown are flexible bands or belts which act as cooling surfaces and enclose or confine the molten metal introduced into the moving mold between them, and they simultaneously move the molten metal progressively toward solidification into forms or products, such as strip, sheet, slab, plates, bars, or billets, hereinafter called the "cast product" or "product being cast". Continuous casting machines employing such flexible bands or belts, often called twin-belt casters, have been pioneered and manufactured for many years by the Hazelett Strip-Casting Corporation of Mallets Bay, Vt. If further information on various aspects of such machines is desired, it can be obtained from the patents assigned to that Company, the assignee of the present invention.
In the introduction, feeding, or charging of molten metal into the moving mold of a substantially horizontal or downwardly inclined continuous casting machine, critical factors for casting metal of acceptable quality and having appropriate surface qualities and surface characteristics for commercial applications are the avoidance of rapid changes in the velocity of the molten metal being introduced, and the avoidance of turbulence in the molten metal, the limiting of exposure of the metal to a reactive atmosphere or other reactive agents, and the provision of favorable interaction between the moving mold surfaces and the metal being confined by these surfaces.
Molten metal handling and distribution equipment, which conveys the molten metal to be cast from the melting or holding furnace to the mold region of the casting machine, is generally designed to avoid restrictions and to limit exposure of the molten metal to an uncontrolled atmosphere, usually accomplished by under-pouring at each transfer. Thus, the molten metal is not poured over an open lip, but instead is drawn well below the surface in the vessel, so as to leave behind surface oxides and most foreign matter. Such under-pouring technique further transfers or introduces the molten metal into the next vessel under the surface of the metal therein, in such a way as to minimize agitation and to avoid contact with atmospheric or oxygen-bearing agents. These strictures and techniques apply generally to the handling of molten lead, zinc, aluminum, copper, iron and steel, and to the alloys of these metals, as well as to other metals. Failure to observe such strictures and techniques may result in the uncontrolled formation of oxides, which tend to adversely affect the metallurgical qualities of the metal being cast, and which otherwise cause difficulty in the molten-metal feeding equipment and in the mold. In certain of these metals, relatively small percentages of oxygen are capable of causing such difficulties. Hydrogen may also become dissolved within the cast metal emanating from the dissociation of atmospheric water vapor molecules resulting from contact with the hot molten metal or from contact with hydrogen-bearing combustion gases. Such hydroggen dissolved, even in small quantities, can cause undesirable porosity. Even nitrogen may be unwelcome, under some conditions.
Oxidation problems within launders, troughs, and tubdishes have been generally solved by under-pouring, together with the use of reducing atmospheres applied to the surface of the molten metal. Such reducing atmospheres are obtained through flames of burning oil or gas which are rendered deficient in the oxygen supplied to them. In the case of aluminum, a protective oxide film will remain quietly upon the surface of an open vessel, when designed so as to minimze agitation, and in this case reducing atmospheres are not required in the preliminary stages of aluminum transfer with under-pouring.
Entrapment of oxides, or other impurities, is less apt to occur in the conventional vertical continuous casting processes, which use a rigid mold that is open at the top and bottom. In those vertical casting processes the pouring into the mold is generally accomplished by under-pouring, and at a relatively slow rate. Such oxides, and other impurities as do form, have time to float to the top, and thus they are prone to remain in the top oxide layer which forms there or to become frozen in the center or core region of the ingot of relatively large cross-sectional area being cast. In this case of vertical casting of large cross-sectional products, the entrapped oxides or other impurities are not likely to be detrimental to, nor render unacceptable, the products being cast.
The situation is quite different and peculiar in the casting of relatively thin, i.e. 1/4 inch (6 mm) to 11/2 inches (38 mm) sections in substantially horizontal or downwardly inclined continuous casting machines. When the mold region is elongated as in twin-belt casters, for example, the continuously moving mold surfaces are normally operated at relatively high linear speeds. Here the problems of entrapment of oxides, or other impurities, can be more serious and can render the product being cast unacceptable.
When casting such relatively thin sections, i.e. 1/4 inch to 11/2 inches, close to the horizontal, the technique of under-pouring for the introduction of the molten metal into the moving mold region of continuous casting machine is usually not practical or feasible, as there is insufficient vertical clearance between the mold surfaces. When casting such relatively thin sections, the molten metal is usually introduced through a semi-sealing nosepiece. As a practical matter this nosepiece must be spaced slightly away from the moving mold surfaces near the entrance to the mold region in order to compensate for the inevitable variables and variations in the entrance to the continuously moving mold. Such spacing from the continuously moving mold surfaces is also needed to allow for the dimensional tolerances involved in the forming and shaping of the refractory material having suitable physical, chemical and thermal properties for the demanding service of handling molten metal. The refractories suitable for this demanding purpose are difficult to shape and maintain within close and consistent operating tolerances.
Thus, the fit between the nosepiece for feeding molten metal and the continuously moving mold surfaces must be relatively loose, with an initial gap of 0.010 inch (0.25 mm) being customary for a new nosepiece. However, this gap, through wear, will tend to widen, especially on the top of the nosepiece. The periodic leakage of most molten metals around the sealing surfaces of the nosepiece is inevitable if the operator of the moving mold attempts to keep the mold region continuously filled up against the nosepiece with molten metal. In other words, it is just usually not practicable to attempt to keep the molten metal in the mold region full up against the nosepiece. Indeed, a gap of about 0.020 inch (0.5 mm) around the nosepiece will generally leak any molten metal of low surface tension, and such metal will readily, quickly solidify or freeze untimely into "fins", causing an undesirable jamming action against the nosepiece, resulting in destruction of the nosepiece.
Consequently, it is usually necessary to avoid filling the mold region so as to avoid back-up of the molten metal up to the nosepiece. Such attempted filling is somewhat more tolerable with aluminum, because of its high surface tension which tends to impede leakage through the gaps. Even with aluminum, however, a "head" of molten metal significantly higher than the upper mold region is to be avoided, because the resultant pressure in the molten aluminum at the gaps near the nosepiece will overcome the surface tension and cause leakage. Therefore, even with aluminum, the operator will often keep the level of molten metal in the mold region no higher than the front lower edge of the nosepiece, so that a considerable gas cavity will be present.
Actually, during the continuous casting, notably of aluminum, with a closely fitting nosepiece, a small gas cavity will persist despite a small head of metal pressure that is slightly higher than any point in the mold region; that is, higher than the location of said residual gas cavity. It is our belief that this phenomenon of an unintended residual gas cavity results in part from the dynamics of the in-feed and from the drag of the moving mold surfaces upon the surface of the molten metal, augmented by surface tension.
Therefore, as a result of intentional operation to avoid any chance for leakage of the molten metal to occur out through the gaps adjacent to the nosepiece or even where not intended, as a result of such dynamic drag phenomenon, there is usually a gas space or cavity within the mold region. This cavity is located in the upper portion of the mold region above the level of the molten metal and adjacent to the front end of the nosepiece.
It will be appreciated that with the nosepiece surfaces positioned within approximately 0.020 of an inch (0.5 mm) near the continuously moving mold surfaces, the operator is not able to ascertain by visual observation the physical status or level of the molten metal at any time in the mold region. Thus, the operator cannot rely upon visual observation to control the level of molten metal or to control the size of the above-described cavity. Novel methods and apparatus for overcoming the difficulties relating to the operator's lack of visible observation for pour level control are described and claimed in U.S. Pat. Nos. 3,864,973 and 3,921,697, whose disclosures are here incorporated by reference. The methods and apparatus of these patents have been successfully applied to twin-belt casters, where they eliminate the need to see physically the level of the molten metal. They have proven practical for control of twin-belt casters in commercial production. Thus, the use of a suitably fitting nosepiece becomes a practical way to introduce metal into the casting region, while maintaining a controlled cavity in the upper portion of the mold region between the nosepiece and the molten metal.
Molten aluminum and aluminum alloys in particular are highly reactive. They can combine with other metals, gases and refractories. For example, in a molten state during continuous casting, aluminum alloys are susceptible to random reaction with or are affected by atmospheric oxygen, water vapor, and trace atmospheric gas pollutants. In the continuous casting of aluminum alloys containing magnesium, random atmospheric contact results in reactions which, in turn, cause oxide spots or streaks on the cast surface, and will also reduce the fluidity of such alloys in a molten state.
The difficulties of uncontrolled oxidation and reaction of the molten metal are compounded in two ways, when relatively thin sections of the order of 1/4 inch (6 mm) to 11/2 inches (38 mm) are being continuously cast. First, there is the cited problem of lack of clearance for means to underpour the metal into the continuously moving mold region, but secondly, the ratio of surface area to volume is increased with such thin sections. As oxidation is generally a surface or interface reaction, oxide formation on such relatively thin continuously cast sections constitutes a greater relative proportion of the product as contrasted with thick sections. Also, with such thick sections it is practical to scalp oxides from the surface of the cast product, but not with the relatively thin sections.
While a portion of the above description has been in terms of twin-belt casting machines, the same problems occur with other types of continuous casting machines in casting relatively thin sections in a horizontal or downwardly inclined mode.